'We're terrified of the British press' ... Tegan and Sara. Photograph: Sarah Lee

'Those are the rules," Sara says fiercely. Her twin sister, Tegan, holds up a laminated card. "Yes," Sara continues, "we have rules on laminates for our tour. Rule number one is: no shoes on the bus, not even to 'just get something from my bunk'." She softens. "I just really believe in keeping people healthy and safe and sane."

Tegan and Sara, 27-year-old twin sisters and purveyors of smart, brilliant pop, are ostensibly here today to discuss their fifth album, The Con. A brief spell in their company, however, sees them dwelling not just on the record, but on the works of Susan Faludi, the socio-economic divide in their native Calgary, and those laminated tour bus rules in full. "I do not want to see inside your bunk!" Sara continues sternly. "And if you pee on the seat you're obviously not old or mature enough to use a toilet."

These rules have been well-honed over almost a decade of touring. The sisters began writing songs at the age of 13, using their school's recording equipment to produce demos. A couple of years later, a triumph at a local battle-of-the-bands contest, and an independently released record, Under Feet Like Ours, caught the attention of Neil Young and his label, Vapor Records. Vapor released their first major album, This Business of Art, and Tegan and Sara went on to tour with Young himself. Since then they have recorded two more highly acclaimed albums, toured with acts as diverse as the Killers, Rufus Wainwright and the Pretenders, and seen one of their songs, the strutty, synthed-up Walking With a Ghost, covered by the White Stripes.

The Con is their first album for Warner Music, and was recorded in Portland, Oregon, home to their producer, Chris Walla, the guitarist/keyboardist with the US indie band Death Cab for Cutie. The record is their most confident to date, in part a reflection of a longer recording schedule: they took a positively luxurious three months to record it, as opposed to the 10 days they spent on This Business of Art. For the first time, they play almost all the instruments on the record themselves, and assisted Walla with the production.

"I think the record just reflects our styles," says Tegan, "which some people embrace, and some people say it's messy and cluttered and out of time ... Well yeah, that's kind of us!" Walla, she says, encouraged them to edge away from a perfect pop sheen towards something more ramshackle and gutsy. "It was all about capturing the emotion and the moment," she says. "If other people had played the guitar solos it would sound more polished and poppier, but it would be missing the heartbreak and the passion of the original recording."

They view the album more like a book than a record. "It was recorded linearly and it has a beginning and a middle and an end," Sara says. The title was Tegan's idea. "I think that the reason for it has evolved," she says. "It just started to feel like everything's a bit of a con. For me, during this record, I was projecting this really happy extroverted image, but inside I was sad. And Sara was writing a lot about anxiety and marriage and death and commitment, and then you do all these stereotypical things, you buy a house and in the end it's ..."

Sara interjects crisply: "Well, it prevents you from thinking about what the reality is, which is that we're all just shuffling towards death and we're all going to lose everything."

The sisters now live in distant parts of Canada - Tegan in Vancouver, Sara in Montreal. But this has not, they stress, hindered their songwriting process. "It's the same way we wrote when we were in the same city," Tegan explains. "We email the songs to each other." In the interests of equality, they divide the album to ensure each sister has the same number of songs. "We don't have sibling rivalry, uh-uh," says Tegan. "It's all about being equal. You have to carry the same weight!"

It is not immediately obvious to the listener that the tracks have been written by two different people, and yet the sisters say their songwriting styles are very different. "I think Tegan has more of a traditional songwriting style," Sara says. "She likes very exposed lyrics, whereas I like it to be more vague - I like songs that are surprises. Tegan's songs translate very well live, whereas my songs are a little more prickly. When you listen to each other's demos they're very different, but when you put them together ..." Tegan darts in to finish her sister's sentence: "... we Tegan and Sara-fy them!"

They are routinely struck, Tegan says, by how warmly people react when they discover their accents are not American. "The perception of Canada when we tour internationally ..." She starts to laugh. "People are like, 'Oh, Canada! You all ride unicorns! And you have a prime minister that gives birth to you!' It's not like you get a hockey-bag of money every day. It's not quite like that. But there is a positivity. And we have intense funding for the arts. You just feel like your government is on your side - I can be mad at them, but I don't fear my government. Whereas in America it feels like people really are afraid of their government."

"It's very 50s right now [in America]," adds Sara. "And there's this return to old-fashioned gender roles. I just read the new Susan Faludi book about how post-9/11 gender roles have shifted, how women should raise the family and men should protect the country and the family. It's a really interesting book." So interesting, in fact, that she has given a copy of it to everyone they work with. "It's strange," she says. "We were raised by a single-parent mom, so we have a very feminist upbringing, but Tegan and I have been very drawn to working with men in business. And yet we have this divide between us, males and females - we don't really understand each other."

When Tegan and Sara tour the US, they say, they have to make a special effort to temper their language. It does not come easy. "Apparently, the first word I ever said was 'fuck'," Tegan laughs. They're a little cautious of media exposure here in Britain, too. The twins are both lesbians, and more column inches have generally been devoted to this fact than to discussing their music.

"For the first seven years we were really terrified of the British press," Tegan says. "There are certain aspects of the media and culture here that we don't support - the celebrity and tabloid culture. It's really depressing, and it demoralises what we do as artists. And America's turning into it." Sara nods. "If you turn on CNN, they're talking about Paris Hilton. If you turn on the Canadian news they're talking about speed bumps. And I'm like, that's great! Speed bumps are what I want to know about, not Paris Hilton."

Today, Tegan is a little nervous about a gig the night before, at which she launched an expletive-ridden tirade against the NME. "We were just trying to be funny," she says. "We were just trying to transfer to our fans that it doesn't matter if you're cool or you have the right haircut or whatever the review said of your record. It's the fact that you're connecting with the people around you." The rant earned her a round of applause. "I thought, 'Oh yeah, we're in London, the only place you can be clapped for being rude.'"

But she is worried the music magazine will retaliate."We don't worry about press anywhere else," she says, half-worried and half-emboldened. "But when we come here we worry about it. And for so long they were unsupportive." She lifts her chin just a little, and raises her voice. "But we're getting more confident now."

· Tegan and Sara play Koko, London (0870 432 5527), on March 5. The Con is out now on Warner Music